Format-preserving encryption is typically used to encrypt data when it is necessary to maintain the data's format for reasons such as validation. One characteristic of format-preserving encryption is that its encrypted data is visually indistinguishable from unencrypted data of the same format. For example, format-preserving encryption of the email address alice@ibm.com may result in the value kfhsu@dfj.bul, which also looks like a valid email address.
Some applications that access data need to know whether the data are encrypted or not. Where the data are stored in a database, one solution would be to add one or more additional fields to the database to indicate which data are encrypted, although this would incur additional storage overhead. Another possibility would be to add an indicator to the data after it is encrypted, to identify it as such. However, this is not possible where doing so would violate format-preservation requirements.